Blocking ABCC4 function in human platelets coupled with PDE inhibition markedly attenuates aggregation. (A) Blocking of human ABCC4 with MK571 reduces the rate and extent of platelet aggregation. (B) The amount of ATP released from platelets is reduced after ABCC4 inhibition. (C) Human platelet aggregation by collagen is strongly reduced by the combination of the PDE2 inhibitor EHNA and the ABCC4 inhibitor MK571. (D) Lag time (in minutes) in collagen-mediated platelet aggregation after the addition of either the PDE2 inhibitor EHNA, the ABCC4 inhibitor MK571, or both in combination. (E) A representative immunoblot of whole-cell lysates prepared from human platelets treated with EHNA, MK571, or EHNA+MK571 for 30 minutes. Blots were probed with p-Vasp (Ser 157 or Ser 239) and total Vasp antibodies.